1. Field of the Invention
The present invention relates to a laser emitter and a laser scanning device suitable for an image exposure device that is provided in an image forming apparatus such as an electrophotographic copying machine or printer to expose a photosensitive body to light.
2. Related Background Art
A description is given on background art of the present invention with reference to FIGS. 9 to 14. FIG. 9 shows an image forming apparatus for printing a color image. The image forming apparatus has an independent image bearing member (hereinafter referred to as photosensitive drum) 120 for each of four colors, that is, yellow, magenta, cyan, and black. The photosensitive drum 120 is an electric conductor with a photosensitive layer formed thereon by application, and forms an electrostatic latent image upon receiving laser light that is emitted from a scanning optical device. The image forming apparatus also has: a scanning optical device 121 for radiating laser light in accordance with image information that is sent from an image reading device (not shown), personal computer or the like; a developing unit 122 for forming a toner image on a photosensitive drum using toner which is charged by friction; an intermediate transfer belt 123 for carrying the toner image on the photosensitive drum to a sheet of transfer paper; a sheet feeding cassette 124 for storing sheets of paper to which toner images are to be transferred; a fixing unit 125 for fixing the transferred toner images to the paper by heat; a sheet delivery tray 126 on which sheets of transfer paper with images fixed thereon are stacked; and a cleaner 127 for cleaning toner that remains on the photosensitive drum.
To form an image, the photosensitive drum 120 is irradiated with laser light which is emitted from the scanning optical device 121 in accordance with image information. This causes a charger to charge the photosensitive drum 120, which then forms an electrostatic latent image. After that, toner charged by friction in the developing unit 122 adheres to the electrostatic latent image to form a toner image on the photosensitive drum 120. The toner image is transferred from the photosensitive drum 120 to the intermediate transfer belt 123, and then re-transferred to a sheet of paper that has been fed from the sheet feeding cassette 124 which is placed in a lower part of the main body of the apparatus. An image is thus formed on the paper. The sheet of paper to which the image is transferred is sent to the fixing unit 125 to fix toner and is discharged onto the sheet delivery tray.
FIG. 10 is a diagram of an image forming portion of FIG. 9 and, since the image forming portion has a symmetrical shape, the symbols used in FIG. 9 are shown on only one side of the unit. The scanning optical device 121 of FIG. 9 emits laser light in accordance with image information, so that an electrostatic latent image is formed on the photosensitive drum 120 by the laser light after the laser light passes through a rotary multi-facet mirror 128, fθ lenses 129 and 130, turn-back mirrors 131a to 131d, and a dust-proof glass enclosure 132. The rotary multi-facet mirror (hereinafter referred to as polygon mirror) 128 is provided for deflection scanning of the laser light. The fθ lenses 129 and 130 are provided to let the laser light run at a constant speed and form a spot on the drum. The turn-back mirrors 131a to 131d are for reflecting the laser beam in a given direction. The dust-proof glass enclosure 132 is provided to protect the scanning optical device 121 from dust. The scanning optical device 121 is placed near the photosensitive drum 120 with the advent of the fashion of scanning optical devices of nowadays which have stopped irradiating photosensitive drums from afar as image forming apparatuses themselves have been reduced in size. As shown in FIG. 10, the scanning optical device 121 employs a method of using one polygon motor unit to irradiate four photosensitive drums, and forms two scanning groups for irradiating opposing faces of the polygon mirror 128 with plural laser beams each. The turn-back mirrors 131a to 131d are used to make the unit compact, and are each a mold lens in which two lenses are pasted together or two light paths are unitarily formed in order to let laser beams on two different light paths image on a photosensitive drum. In this collimating optics, a deflection plane for deflection scanning of laser light is necessary for each light path. Therefore, a thick polygon mirror or a two-stage structure polygon mirror is employed in this system.
In contrast to an optical system that uses such a polygon mirror as the one described above, there is an optical system as the one shown in FIG. 11 which uses a thin polygon mirror 133 to obtain a thinner shape. In this optical system, laser beams enter and exit the polygon mirror 133 at different angles in a sub-scanning direction so that beams for irradiating photosensitive drums are split at a point where the beams are spaced at regular intervals from one another. Laser light is run for deflection scanning with the use of the polygon mirror 133 and then is transmitted through common fθ lenses 135 and 136. After that, each laser beam passes through two turn-back mirrors and one concave mirror 134b or 134e and is applied to its associated photosensitive drum. To split laser light, a laser beam that is run on the lower side in FIG. 11 by deflection scanning is reflected upward in FIG. 11 toward an inner photosensitive drum by a turn-back mirror 134d, which is placed at a point in the light path of the laser beam, so that the laser beam intersects a laser beam that is run on the upper side by deflection scanning, and the reflected light is then radiated onto the photosensitive drum by turn-back mirrors 134e and 134f, which are placed in an upper part of the scanning optical device. In the optical system of FIG. 11 where light is incident obliquely, the fθ lenses, having a refractive power in the main scanning direction, work like the collimating optics described above and therefore can be arranged as in the collimating optics. On the other hand, in principle, it is difficult for the fθ lenses of FIG. 11 to assuredly condense light in the sub-scanning direction on a photosensitive drum since light enters the lenses obliquely with respect to the optical axes of the lenses.
It is for this reason that the concave mirrors 134b and 134e are necessarily provided to condense laser beams in the sub-scanning direction after each laser light is split (the same effect is obtained if these concave mirrors are replaced by third imaging lenses each having a refractive power in the sub-scanning direction).
There has been proposed also a modification of the above oblique incident optical system that needs to place four imaging optical elements in total in one group downstream of the polygon mirror. The modified oblique incident optical system is reduced in number of optical elements, and has an fθ lens as a second imaging lens placed downstream of the point where each laser light is split. This structure needs the second imaging lens for each laser light but no concave mirror or third imaging lens, and is therefore capable of condensing laser light on a photosensitive drum with three imaging optical elements. As a result, a scanning optical device having this structure is smaller in size than the above-described oblique incident optical system that uses concave mirrors or third imaging lenses.
Such an optical system where light in a sub-scanning direction is incident obliquely has four laser light sources corresponding to four photosensitive drums. The symmetrical optical system shown in FIG. 11 are composed of two lens barrel portions 153 and 154, which hold two light sources 151 and 152, respectively, two collimator lenses 155 and 156, and two electric circuit substrates 157 and 158 for causing emission of laser light. Those laser light sources are placed side by side in the sub-scanning direction at a distance as shown in a sectional view in the sub-scanning direction of FIG. 13, or placed off the sub-scanning direction with the use of a turn-back mirror 159 as shown in a top view of FIG. 14.
However, the structures of FIGS. 13 and 14 both have problems. To place the laser light sources side by side in the sub-scanning direction at a distance as shown in FIG. 13, the length of the incident optical system from the laser light sources to the polygon mirror has to be set long since a short distance between the laser light sources causes the lens barrel portions to interfere with each other. This increases the size of the scanning optical device.
The structure of FIG. 14 which uses the turn-back mirror to place the laser light sources off the sub-scanning direction is free from the above problem of increasing the scanning optical device in size, but the use of the turn-back mirror increases the number of parts and accordingly cost.